This might cause some issues in some cases, as mentioned here:
#3742 (comment)

We could use wp_remote_get() if the $image_url is prefixed by http:// or https:// however would need to use file_get_contents() for all other protocols and image paths anyway (ie. /home/.../image.jpg or custom-file-wrapper://path/image.jpg)

Ah, thanks, I hadn't seen that.

The function name does explicitly have from url in the name, so specifying a local path doesn't seem like it would be done often? But I'd be happy to add in caveats for that as well for better compat and fewer http requests.

Also other possible place --

Works fine with URLs, images are recognized, even with no extensions. Non images return false, which is correct too.
Loading through filesystem isn't working though.

I tested with something like

add_action( 'wp_footer', function() {
    $ton = new Tonesque( 'http://example.tld/wp-content/uploads/2029/08/image.jpg' );
    var_dump( $ton->grab_color() );
} );

and it works, you get the array with r, g and b keys. But, if you replace the URL with a path like /Users/someuser/Sites/someserver/wp-content/uploads/2029/08/image.jpg it doesn't work.

Personal opinion: I think we should add another method imagecreatefromfile instead of complicating things in imagecreatefromurl

It appears that in PHP7, list() must be fed an integer-keyed array. I received the following when loading up Tonesque with a local file path:

Notice: Undefined offset: 0 in /home/jetpack-single.ftlwp.com/wp-content/plugins/jetpack/_inc/lib/tonesque.php on line 69 Notice: Undefined offset: 1 in /home/jetpack-single.ftlwp.com/wp-content/plugins/jetpack/_inc/lib/tonesque.php on line 69 Warning: imagecreatefromstring(): Empty string or invalid image in /home/jetpack-single.ftlwp.com/wp-content/plugins/jetpack/_inc/lib/tonesque.php on line 76

I just pushed a commit that should resolve this. Otherwise, everything LGTM and tests good.

LGTM! 🐑 Tested again with Ryu theme and this code and it works fine now

add_action( 'wp_footer', function() {
    $ton = new Tonesque( 'http://example.tld/wp-content/uploads/2029/08/image.jpg' );
    var_dump( $ton->grab_color() );

    $ton = new Tonesque( '/Users/joedoe/Sites/joeserver/wp-content/uploads/2029/08/image.jpg' );
    var_dump( $ton->grab_color() );
} );